A variety of filters have hitherto been proposed for filtering a tone signal of a specified frequency. Of filters of this type, a conventional filter used for digital processing, that is, a digital filter will be described with reference to FIG. 1. As diagrammatically shown in FIG. 1, an input terminal 1 is connected to one end of each of a plurality of capacitors via a resistor 2. In this example, four capacitors 3, 4, 5 and 6 are employed. One end of each of the capacitors 3 to 6 is connected to one end of an input resistor 7 the other end of which is connected to a high gain amplifier 9. A feedback resistor 8 is connected across the amplifier 9 and a amplified signal is delivered from an output terminal 10. The other end of each of the capacitors 3 to 6 is connected to respective collectors of switching transistors 11, 12, 13 and 14. Signals 19, 20, 21 and 22 of different phases are applied to respective bases of the transistors 11 to 14 via resistors 15, 16, 17 and 18 during a time interval which is equal to 1/4 of the period of a waiting frequency .function.o. The transistors 11 to 14 are turned on when these signals are at high level.
In operation, an incoming signal from the input terminal 1 sequentially charges, via the resistor 2, capacitors 3 to 6 associated with respectively turned on transistors 11 to 14 during only the transistor conduction time interval. When the period of the incoming signal coincides with the switching period of the transistors 11 to 14, each of the capacitors 3 to 6 is charged each time from a corresponding same portion of the signal period. Consequently, the input signal is divided on the time division basis and an average voltage of the input signal within a time division segment is stored in each of the capacitors 3 to 6. Within a time interval subsequent to the charging interval, the incoming signal current is converted into a voltage at the amplifier 9 and an output voltage is produced therefrom.
When the period of the incoming signal is not synchronized with the switching period of the transistors 11 to 14, each of the capacitors 3 to 6 is charged by a lower (or higher) voltage than the previous, synchronized charging voltage so that the charge of the capacitor is decreased (or increased) by discharging (or charging) via the resistor 2. In such a case, the incoming signal current flows to the earth via the capacitors 3 to 6 and the amplitude of the converted output voltage from the amplifier 9 is decreased.
In this manner, only the input signal having the period which is in synchronism with the switching period of the switching transistors can always be passed on and a stable output signal develops at the output terminal 10. This type of filter having N signal branches is called an N-path filter and is known from Bell System Technical Journal, September, 1960, pp 1321-1350, for example. Recent development of MOS transistor technology permits replacement of the switching transistor 11 to 14 in FIG. 1 with FET analog switches, giving a solution to problems of the collector--emitter saturation voltage.